The invention relates to a controller for a motor, and particularly, a controller having a load overload device.
To comply with the National Electrical Code, NFPA 70, electric motors larger than one horsepower are required to use overload devices. The overload devices are intended to protect electric motors and branch circuits from undue heating caused by excessive current.
Induction electric motors are widely used. Induction motors are generally specified by a variety of rated capacity parameters such as torque, horsepower, current, voltage, frequency, temperature, starting time, and the like. While induction motors have a simple robust physical design, induction motors rely on complex nonlinear relationships to function. For example, when an inductive motor is first turned on to drive a machine, the inductive motor can draw an additional amount of current to provide additional torque to drive the machine. The additional amount of current drawn is generally referred to as an inrush current that can last for a few seconds. The inrush current can sometimes be ten times higher than that of a normal running current. The additional torque can sometimes be three times that of a normal operating torque. After the driven machine reaches a normal operating state or speed, the current drawn will drop below the name plate current value or rated current capacity.
However, when the machine being driven becomes jammed or impaired in some manner, the motor will draw additional current or power to churn out additional torque in an attempt to move the machine. When the amount of the operating current drawn by the motor exceeds a certain rated amount, an overload device associated with the machine will trip. For example, an overload of about 125 percent of the rated current of the motor for about 600 seconds, or about 600 percent of the rated current of the motor for about ten seconds will trip the overload device. However, the additional torque can last for a period of time before the overload device trips. While the machine being driven is jammed or impaired and before the overload device trips, the jammed machine can destroy any jammed material or itself.